The present invention relates to forming holes in articles, such as gas turbine engine components, and more particularly to an electrode for use in an electrical discharge machining (EDM) device.
Gas turbine engines operate at extremely high temperatures for increased efficiency. Stationary vanes, disposed between rings of moving blades within the turbine section of the engine direct and stabilize high temperature gas flow from one stage of moving blades to the next. Direct exposure to this high temperature gas, however, detrimentally affects the vanes and blades by causing component distortion and even melting in extreme cases.
Internal cooling techniques have been developed to maintain the temperature of the blades and vanes within design limits while operating at high temperatures. The outer surface of engine components is typically cooled with high pressure cooling air from the compressor section of the engine to provide film cooling. In this method, a layer of cool air is flowed between the high temperature gases and the external surfaces of the engine components. The layer of cooling air is formed by passing the cooling air through a series of small holes in the component which are formed in a predetermined pattern. The resulting film of air reduces component surface temperature thereby deterring component distortion. Engine efficiency is also increased because higher turbine inlet temperature ranges are possible.
Many processes exist to form cost effective, high quality cooling holes in gas turbine engine components. One such process is electrical discharge machining (EDM). EDM is a well-known process for producing holes or other openings in metals in which current discharges are utilized to erode metal. For example, by pulsing a direct current between a positively charged work piece (anode) and a negatively charged electrode (cathode), a spark discharge is produced. The current occurs when the potential difference between the electrode and the work piece, both in contact with a dielectric fluid, is great enough to breakdown the dielectric fluid and produce an electrically conductive channel. Upon application of a voltage, a current flow results with enough heat energy to melt and erode the work piece. This process has application in the machining of small, deep, odd-shaped holes which are cumbersome, if not impossible, to produce by other means.
A conventional EDM method for producing diffusion holes in engine components uses a planar copper electrode or “EDM comb” which is typically manufactured by stamping and coining. The teeth of the electrode are small diameter elongated ends that form a hole shape defined by electrode, with allowance for electrode overburn and EDM electrode erosion.
Although the above EDM method is successful, limitations exist. One such limitation is that conventional planar EDM combs are constrained to a straight line of holes, which are only applicable to a relatively straight airfoil sections.
Accordingly, it is desirable to provide an EDM electrode which can produce high quality holes which can be applied to highly contoured airfoil sections of an airfoil in an efficient and effective manner.